Browsing by Subject "Nanofluid"
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Item A Survey of Measurements, Models, and Explanations of the Effect of Nanoparticles on the Specific Heat of Various Nanofluids(2014-12-01) Shan, XiaowanA survey was conducted to determine if any unified models and/or explanations exist that adequately predict the change in specific heat of a nanofluid as a function of concentration of nanoparticles. The papers and previous studies about the specific heat of nanofluids were collected and reviewed. The existing models and experimental data were summarized and compared. Through the investigation, only four different models were discovered, and two types of experimental result of the specific heat of nanofluids were found. Then, the four models were used to predict the change of the specific heat of nanofluids and compare with different selected sets of experimental result. This was intended to determine if the models are suitable for predicting the results of different sets of published results. The published results can be categorized into two types: a decrease in specific heat resulted from the addition of nanoparticles; and an initial increase in specific heat as nanoparticles were added followed by a decrease as the concentration of particles was increased above a particular value. The comparisons showed that there is no unified model that can predict or theory that can explain all the experimental results. The mechanisms of the enhancement of the specific heat of nanofluids were presented. The results indicated that the enhancement of the specific heat of nanofluids was caused by the high specific surface area of nanoparticles. The specific surface area is a property of a material which is the total surface area of a material per unit of mass. A higher specific surface area of a material results in an increase of the heat transfer velocity between the material and surroundings. Further, two novel explanations of the change in the specific heat at a nanoscale level have been proposed. Additionally, suggestions at a nanoscale level for future research of the specific heat of nanofluids have been recommended.Item Experimental Techniques to Study the Effects of Nanoparticle Additives on Heterogeneous Monopropellant Combustion(2014-04-01) McCown, Kenneth WoodrowThis thesis presents the development of techniques used to investigate the combustion behavior of liquid monopropellants with or without additives, with a focus on nano-scale particles as burning rate modifiers in nitromethane. The linear burning rates of these mixtures were measured in a constant-volume system at chamber pressures ranging from 3 to 14 MPa, all without direct observation of the propellant burning front. Distinct differences in burning rates were observed between burns using a quartz-lined cavity and those employing plain carbon steel. Several analytical models and numerical approximations were used to estimate the temperature profiles of quartz, steel, and layered strand burner tubes, indicating that the higher burning rates measured in the steel cavity were likely caused by a combination of heat transfer and catalytic effects. The close match between the burning rates of neat nitromethane gathered in this study and those taken from recent studies utilizing optical systems proves the utility of the author?s method, while the consistently measured burning rates of the various nitromethane-based nanofluids prove the versatility of the same method when extended to tests on suspended-particle mixtures. Nano-scale aluminum was used to increase the overall energy density of propellant mixtures, fumed silica powder was used to increase the mixture thickness and encourage aluminum suspension, and nano-scale titania was also included based on its previous use as a burning rate modifier in solid propellants. The silica loading was varied from 1% to 3% by weight, aluminum loading was varied from 5% to 13.5% by weight, and titania was added at 1% by weight. A comprehensive settling study was used to characterize the stability of numerous propellant mixtures, quantifying the particle settling rates of unstable mixtures while subsequently eliminating this instability from all burned configurations. This thesis observed a wide variety of particle effects on the combustion behavior of nitromethane; some of these trends were previously observed by other research groups, while several burning rate effects were observed by the current author for the first time. These novel behavioral trends included an increase in propellant pressure sensitivity over the tested 3- to 14-MPa range for mixtures that included 3% silica by weight, and an even more dramatic increase in pressure sensitivity and linear burning rates was observed only at chamber pressures above 8 MPa for propellants that included 1% titania by weight without silica. The various performance trends uncovered and techniques developed through this study have already been applied to new mixtures based on more exotic compounds, utilizing the lessons learned herein as a springboard to greatly expand the range of propellants currently tested at Texas A&M University.Item Microfluidic Investigation of Tracer Dye Diffusion in Alumina Nanofluids(2012-10-05) Ozturk, Serdar 1979-Nanofluids, a new class of fluids engineered by suspending nanometer-sized particles in a host liquid, are offered as a new strategy in order to improve heat and mass transfer efficiency. My research was motivated by previous exciting studies on enhanced mass diffusion and the possibility of tailoring mass transport by direct manipulation of molecular diffusion. Therefore, a microfluidic approach capable of directly probing tracer diffusion between nanoparticle-laden fluid streams was developed. Under conditions matching previously reported studies, strong complexation interactions between the dye and nanoparticles at the interface between fluid streams was observed. When the tracer dye and surfactant were carefully chosen to minimize the collective effects of the interactions, no significant change in tracer dye diffusivity was observed in the presence of nanoparticles. Next, adapting tracer dyes for studies involving colloidal nanomaterials was explored. Addition of these charged tracers poses a myriad of challenges because of their propensity to disrupt the delicate balance among physicochemical interactions governing suspension stability. Here it was shown how important it is to select the compatible combinations of dye, nanoparticle, and stabilizing surfactant to overcome these limitations in low volume fraction (< 1 vol%) aqueous suspensions of Al2O3 nanoparticles. A microfluidic system was applied as a stability probe that unexpectedly revealed how rapid aggregation could be readily triggered in the presence of local chemical gradients. Suspension stability was also assessed in conjunction with coordinated measurements of zeta potential, steady shear viscosity and bulk thermal conductivity. These studies also guided our efforts to prepare new refrigerant formulations containing dispersed nanomaterials, including graphene nanosheets, carbon nanotubes and metal oxide and nitride. The influence of key parameters such as particle type, size and volume fraction on the suspension's thermal conductivity was investigated using a standard protocol. Our findings showed that thermal conductivity values of carbon nanotube and graphene nanosheet suspensions were higher than TiO2 nanoparticles, despite some nanoparticles with large particle sizes provided noticeable thermal conductivity enhancements. Significantly, the graphene containing suspensions uniquely matched the thermal conductivity enhancements attained in nanotube suspensions without accompanying viscosity, thus making them an attractive new coolant for demanding applications such as electronics and reactor cooling.